Metazoa: Animal Life and the Birth of the Mind

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Dedicated to all who lost their lives in the Australian bushfires of 2019–20, and to the people who fought the fires

And let me in this place movingly admonish you, ye ship-owners of Nantucket! Beware of enlisting in your vigilant fisheries any lad with lean brow and hollow eye; given to unseasonable meditativeness.… Why, thou monkey, said a harpooneer to one of these lads, we’ve been cruising now hard upon three years, and thou hast not raised a whale yet. Whales are scarce as hen’s teeth whenever thou art up here. Perhaps they were; or perhaps there might have been shoals of them in the far horizon; but lulled into such an opium-like listlessness of vacant, unconscious reverie is this absent-minded youth by the blending cadence of waves with thoughts, that at last he loses his identity; takes the mystic ocean at his feet for the visible image of that deep, blue, bottomless soul, pervading mankind and nature; and every strange, half-seen, gliding, beautiful thing that eludes him; every dimly-discovered, uprising fin of some undiscernible form, seems to him the embodiment of those elusive thoughts that only people the soul by continually flitting through it. In this enchanted mood, thy spirit ebbs away to whence it came; becomes diffused through time and space; like Wickliff’s sprinkled Pantheistic ashes, forming at last a part of every shore the round globe over.

—HERMAN MELVILLE, Moby-Dick; or, The Whale

1

PROTOZOA

Down the Steps

You walk ten steps down on a stairway shaped from breakwater rocks straight into the water, which is flat and still, right at the top of the tide. Sound recedes with gravity and light fades to soft green as you dip beneath the surface. All you can hear is your breathing.

Soon you are in a sponge garden, in a jumble of shapes and colors. Some of the sponges have the form of bulbs or fans, growing upward from the seafloor. Others spread sideways over whatever they find, in an irregular encompassing layer. Amid the sponges are what look like ferns and flowers, and also ascidians (with a silent c), pale pink spout-like structures with enamel patterns inside. The spouts resemble the downward-curved air funnels on the decks of ships, though these spouts face in every direction. They are covered by all manner of tangled life, often so encrusted that they appear to be part of the physical landscape in which things live rather than organisms in their own right.

But the ascidians make small shifts, as if asleep and half sensing you as you pass. Occasionally, and always startling me a little, an ascidian body half-collapses in place and visibly expels the water held inside the animal, as if with a shrug and sigh. The landscape comes to life and makes its own comment as you go by.

Among the ascidians are anemones and soft corals. Some corals take the form of a cluster of tiny hands. Each hand has the regularity of a flower, but a flower that grasps at the water around it. They clench and slowly open again.

You are swimming through something like a forest, surrounded by life. But in a forest, most of what you encounter is the product of a different evolutionary path: the plant path. In the sponge garden, most of what you see are animals. Most of those animals (all except the sponges themselves) have nervous systems, electrified threads that stretch through the body. These bodies shift and sneeze, reach and hesitate. Some react abruptly as you arrive. Serpulid worms look like tufts of orange feather fixed to the reef, but the feathers are lined with eyes, and they vanish if you come too close. One can imagine being in a green forest, and finding the trees sneezing and coughing, reaching out hands, glimpsing you with invisible eyes.

This slow swim out from shore is showing you remnants and relatives of early forms of animal action. You are not swimming into the past—the sponge, ascidian, and coral are all present-day animals, products of the same span of evolutionary time that produced humans. You are not among ancestors but far-removed cousins, distant living kin. The garden around you is made of the topmost branches of a single family tree.

Farther out and under a ledge is a tangle of feelers and claws: a banded shrimp. Its body, partly transparent, is just a few inches long, but antennae and other appendages extend its presence at least three times as far. This animal is the first I’ve mentioned that might see you as an object, rather than responding to washes of light and looming masses. Then a bit farther still, on top of the reef, an octopus is stretched out like a cat—a very camouflaged cat—with several arms extended and others curled. This animal watches you, too, more overtly than the shrimp, raising its head in attention as you pass.

Matter, Life, and Mind

Something was dredged from the depths of the North Atlantic by HMS Cyclops in 1857. The sample looked like seafloor mud. It was preserved in alcohol and sent to the biologist T. H. Huxley.*

The sample was sent to Huxley not because it seemed especially unusual, but because of an interest, both scientific and practical, in seafloors at the time. The practical interest stemmed from the project of laying deep-sea telegraph cables. The first cable to span and send a message across the Atlantic was completed in 1858, though it lasted only three weeks, when the insulation failed and the signal-carrying current leaked away into the sea.

Huxley looked at the mud, noted some single-celled organisms and puzzling round bodies, and stored the sample away for about ten years.

He returned to it then with a better microscope. This time he saw discs and spheres of unknown origin, and also a slime-like substance, a transparent gelatinous matter, surrounding them. Huxley suggested that he had found a new kind of organism, of an exceptionally simple form. His cautious interpretation was that the discs and spheres were hard parts produced by the jelly-like matter itself, which was alive. Huxley named the new organism after Ernst Haeckel, a German biologist, illustrator, and philosopher. The new form of life was to be called Bathybius Haeckelii.

Haeckel was delighted with both the discovery and christening. He had been arguing that something like this must exist. Haeckel, like Huxley, was entirely convinced by Darwin’s theory of evolution, unveiled in On the Origin of Species in 1859. Huxley and Haeckel were the leading advocates of Darwinism in their respective countries, England and Germany. Both were also eager to press on to questions that Darwin had been reluctant, beyond a few brief passages, to speculate about: the origin of life and the beginning of the evolutionary process. Did life arise just once on Earth, or several times? Haeckel was convinced that the spontaneous generation of life from inanimate materials was possible, and might be going on continually. He embraced Bathybius as a fundamental form of life, one that might cover large tracts of the deep seafloor; he saw it as a bridge or link between the realm of life and the realm of dead, inorganic matter.

The traditional conception of how life is organized, a picture in place since the ancient Greeks, recognized just two kinds of living things: animals and plants. Everything alive had to fall on one side or the other. When the Swedish botanist Carl Linnaeus devised a new scheme of classification in the eighteenth century, he installed plant and animal kingdoms alongside a third, inanimate realm, the kingdom of rocks, or Lapides. This three-way distinction is still seen in the familiar question, animal, vegetable, or mineral?

By the time of Linnaeus, microscopic organisms had been observed, perhaps first in the 1670s by the Dutch draper Antonie van Leeuwenhoek, who made the most powerful of the early microscopes. Linnaeus included a fair number of tiny, microscopically observed organisms in his classification of beings, putting them in the category of worms. (He concluded the tenth edition of his Systema Naturae, the edition that began the classification of animals as well as plants, with a group he called Monas: body a mere point.)

As biology progressed, puzzle cases began to appear, especially at the microscopic scale. The tendency was to try to put them with either plants (algae) or animals (protozoa), on one side of the boundary or the other. But it was often hard to tell where some new creature belonged, and natural to feel that the standard classification was under strain.

In 1860, the British naturalist John Hogg argued that the sensible thing to do was to cease the shoehorning and add a fourth kingdom for the small organisms, increasingly recognized as single-celled, that are neither plants nor animals. These he called Protoctista, and he placed them in a Regnum Primigenum, or primeval kingdom, that accompanied animals, plants, and minerals. (Hogg’s term, Protoctista, was later shortened by Haeckel to the more modern Protista.) As Hogg saw it, the boundaries between the different living realms were vague, but the boundary between the mineral kingdom and the living was sharp.

The wrangling of categories I’ve described has so far been concerned with life, not with the mind. But life and the mind have long seemed linked somehow, even if their perceived relationship has not been stable. In the framework of Aristotle, developed over two millennia earlier, soul unifies the living and the mental. Soul, for Aristotle, is a kind of inner form that directs bodily activities, and it exists in different levels or grades in different living things. Plants take in nutrients to keep themselves alive—that shows a kind of soul. Animals do this and can also sense their surroundings and respond—that is another kind of soul. Humans can reason, in addition to the other two capacities, and so have a third kind. For Aristotle, even inanimate objects that lack souls also often behave in accordance with purposes or goals, tending toward their natural place.

The overthrow of Aristotle’s picture in the seventeenth century’s Scientific Revolution included a redrawing of these relationships. This involved a hardened conception of the physical—the assertion of a mechanical, push-pull view of matter with little or no role for purpose—and a lifting or etherealization of the soul. The soul, integral to all living nature in Aristotle, became a more rarified, intellectual affair. Souls may also be saved by divine will, permitting a kind of eternal life.

For René Descartes, an especially influential figure in this period, there is a sharp divide between the physical and the mental, and we humans are a combination of both; we are physical and mental beings. We succeed in being both because the two realms make contact in a small organ in our brains. This is Descartes’s dualism. Other animals, for Descartes, lack souls and are purely mechanical—a dog is without feeling, no matter what is done to it. The souls that make humans special are no longer present, even in faint forms, in animals and plants.

In the nineteenth century, the time of Darwin, Haeckel, and Huxley, advances in biology and other sciences made dualism of Descartes’s kind look less and less viable. Darwin’s work suggested a picture in which the divide between humans and other animals is not so sharp. Different forms of life along with different mental powers might arise through gradual processes of evolution, especially by adaptation to circumstances and the branchings that originate species. This should suffice to explain both bodies and minds—if you can get things started.

That was a big if. Haeckel, Huxley, and others approached this part of the problem as follows. They thought there must be a stuff, present in living things, that enables both life and the beginnings of a mind. This stuff would be physical, not supernatural, but quite unlike ordinary matter. If we could isolate it, you could pick up a spoonful of it, and in your spoon it would still be the special stuff. They called it protoplasm.

This might seem an odd approach, but it was motivated in part by close inspection of cells and simple organisms. When people looked inside cells, it seemed that not enough organization was present—not enough parts were different from other parts—for cells to do what they are evidently able to do. What they saw seemed to be just a substance, transparent and soft. The English physiologist William Benjamin Carpenter, writing in 1862, marveled at what single-celled organisms could achieve: the vital operations that one sees carried on by an elaborate apparatus in an animal are instead brought about by a little particle of apparently homogeneous jelly. The particle of jelly is seen laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, and moving from place to place without muscles. This led Huxley, and others, to think that it could not be an intricate organization of ordinary matter that explains living activity, but a different ingredient, one that was inherently alive: organization is the result of life, not life the result of organization.

Against that background, Bathybius seemed extraordinarily promising. It appeared to be a pure sample of the stuff of life, stuff that perhaps arises spontaneously all the time, forming an ever-renewing deep-sea organic carpet. Further samples were examined. Bathybius obtained from the Bay of Biscay was described as being capable of movement. Other biologists were not so sure about this alleged primordial life-form, however, and the growing mass of speculation around it. How was Bathybius staying alive down there? What might it eat?

Then came the Challenger expedition—a four-year project organized by the Royal Society of London in the 1870s that took samples from hundreds of deep-sea sites around the world. The aim was the first comprehensive inventory of life in the deepest waters. The chief scientist on the expedition, Charles Wyville Thomson, was willing to work on the Bathybius question although he was wary of it. No fresh samples were found by the Challenger, and two scientists aboard the ship began, amid some tinkering, to suspect that Bathybius was not alive and not even close to it. With a series of experiments, they showed that Bathybius appeared to be nothing more than the product of a chemical reaction between seawater and the alcohol used to preserve samples, including Huxley’s old sample from the HMS Cyclops.

Bathybius was dead. Huxley acknowledged his error immediately. Haeckel, more committed to Bathybius as a missing link, hung on, unfortunately, for nearly ten more years. But the bridge had failed.

Afterward, some people still held out hope for a bridge of roughly the same kind—a special substance that would link life with matter. But in the years that followed, views of that kind subsided. They were replaced in a slow process of discovery, a process that eventually made living activity no longer mysterious. The resulting explanation of life proceeded in exactly the way that Huxley and Haeckel could not countenance: in terms of the hidden organization of ordinary matter.

That matter is not ordinary in every sense, as we will see, but it is ordinary in its basic composition. Living systems are made of the same chemical elements that make up the rest of the universe, running according to physical principles that extend also into the inanimate realm. We don’t presently know how life originated, but its origin is no longer a mystery of a kind that might make us believe that some extra substance generates the living world.

This has been the triumph of a materialist view of life—a view that permits no supernatural intrusions. It was also the triumph of a view that sees the physical world itself as unified in its basic constituents. Living activity is not explained in terms of a mysterious ingredient, but in terms of intricate structure on a tiny scale. That scale is almost inconceivable. To pick just one example, ribosomes are important parts of cells—the stations where protein molecules are assembled—with a rather complex structure of their own. But over 100 million ribosomes could fit on the period printed at the end of this sentence.

Life, then, has fallen into place. In the case of the mind, much less is resolved.

The Gap

From the late nineteenth century onward, with Darwin’s revolution gathering steam, it seemed hard to maintain a dualist view of the mind like Descartes’s. Dualism makes some sense within an overall picture that locates humans as a unique and special part of nature, close to God in some way. Then all the rest, alive or dead, can be purely material, while we have an added ingredient. An evolutionary perspective on humanity, one that sees continuities between ourselves and other animals, makes dualism difficult, though not impossible, to maintain. This motivates the attempt to develop a materialist view of the mind, one that explains thought, experience, and feeling in terms of physical and chemical processes. The fact that life itself succumbed to a materialist treatment of this kind is encouraging, but it is not clear how much it really helps; it’s not clear what relationship the success of materialism in biology has to the puzzles of the mind.

Looking again at the history, we can distinguish two alternative paths that continue through to the present. Aristotle, as we saw, recognized several different grades of soul, linking plants, animals, and ourselves. What we call mind is viewed as a natural extension, or version, of living activity. Aristotle’s view was not an evolutionary one, but it is not too hard to recast such a picture in evolutionary terms. The evolution of complex life naturally gives rise to the mind, through the growth of purposeful action and sensitivity to the environment.

Descartes, in contrast, saw life as one thing and mind as entirely another. There is no reason, in this second view, to think that progress in understanding life will make much of a difference to problems about the mind.

Over the last century or so, most views in this area have been materialist, but in one respect they have moved close to Descartes. From the mid-twentieth century onward, theorists shifted away from seeing close connections between the nature of life and the mind. This was encouraged by the advance of computers. Computer technology, as it developed from the middle decades of the last century, promised a different bridge between the mental and the physical, a bridge made of logic rather than life. The new mechanization of reasoning and memory—computation—seemed a better way forward. As artificial intelligence (AI) systems developed, some of them started to seem a bit intelligent, but there was little reason to think of them as alive. Animal bodies, it seemed, did not matter very much—they came to appear entirely optional, in fact. Software was the heart of the matter. The brain runs a program, and that program might run on other machines (or things other than machines) as well.

These years also saw a sharpening of the problem of mental and physical. The mind as puzzle was replaced by a more specific conundrum. The new view holds that some of the mind can be fairly readily explained in materialist terms, while another aspect is more resistant. The resistant side is subjective experience, or consciousness. Consider memory, for example. We might find that various kinds of animals have memory; they create traces of the past in their brains, and use those traces later, when working out what to do. It is not too hard to imagine how brains might achieve this. Much of that problem is unsolved, but it certainly looks soluble; we should be able to work out how this side of memory works. But in humans, at least, some kinds of memory also feel like something. As Thomas Nagel put it in 1974, there is something it’s like—something it feels like—to have a mind. There is something it feels like to remember a good experience, or a bad one. The information-processing side of memory, the ability to store and retrieve useful information, might either be accompanied by this additional feature or not. The hard part of the mind-body problem is explaining that last side of our mental lives, explaining in biological, physical, or computer-based terms how felt experience can exist in the world.

This problem is often still approached through a range of classic options. The main divide has materialist (or physicalist) views on one side, and dualism on the other. More radical possibilities are also entertained. Panpsychism holds that all matter, including the matter in objects like tables, has a mental aspect to it. This is not the idea that the entire universe is made of experience—that is idealism. Instead, a panpsychist accepts the physical layout of the world as it appears, but adds that the material that makes up that world always has a side to it that is faintly mind-like. This mind-like side of matter gives rise to experience and consciousness, once some of that matter is organized into brains. Despite its apparent extravagance, panpsychism has serious defenders. Thomas Nagel, who I mentioned above, argues that panpsychism should be kept on the table as an option, because every view has significant problems and panpsychism’s problems are no worse than others’. Ernst Haeckel, in the post-bathybius years, was also attracted to panpsychism. Huxley was attracted to another unorthodox view. He suspected that conscious experience might be an effect of material processes, but never a cause of them. This is an unusual kind of dualism, and it also has defenders today.

Something that is vivid in the wild sweep of these alternative views of the universe, and visible also in more mundane discussions, is a huge diversity in ideas about where minds are to be found. For some, mind is everywhere, or nearly everywhere. For others, it is confined to humans and perhaps a few animals similar to us. One person will look at a paramecium, a single-celled organism, swimming vigorously through a film of water and say: What is going on in that creature is enough for it to have feelings. The paramecium is responsive, and has goals. On a tiny scale, it has experience. Another person will not merely dismiss the paramecium, but will look at a complex animal, like a fish, and say: There is probably no feeling there at all. The fish has a lot of reflexes and instincts, and some fairly complicated brain activity, but all of this activity is going on in the dark. If this second person is wrong, why are they wrong? If panpsychism is also wrong and there is no hint of feeling in a grain of sand, why is that wrong? Might things be that way? There often seems to be a kind of arbitrariness in the situation. People can say whatever they like. If I were to guess where most people stand at the moment, when asked which living things around them have experiences, I would conjecture that a common answer is yesfor mammals and birds, perhaps for fish and reptiles, and no for everything else. But if someone insists on pushing further out (to ants, plants, and paramecia) or pulling further in (mammals only), the discussion rapidly gets a bit untethered. How could we possibly work out who is right?

This sense of arbitrariness is related to something the philosopher Joseph Levine has called the explanatory gap. Even if we come to be pretty sure that the mind must have a purely physical basis, with nothing added, we would also want to know why this physical setup gives rise to this kind of experience, rather than something else. Why does it feel like this to have a brain of the particular kind you have, going through the processes it is going through right now? Even if the difficulties faced by other views convince us that materialism has to be true, it’s hard to see how it’s true, how things could be this way.

That is the cluster of problems I want to address in this book. The aim is not to answer Levine’s questions about particular experiences—which activities of the brain are involved in seeing color or feeling pain. That is a task for neuroscience. The aim, instead, is to make sense of why it feels like something to be a material being of the kind that we are. That we is intended to be rather broad; my main target is not the intricacies of human consciousness, but experience in general, something that might extend to many other animals. I want to address these questions about experience in a way that reduces the sense of arbitrariness I described above—the feeling that you could say yes to bacteria, no to birds, whatever strikes you.

The approach I take to the mind-body problem is biological, and one that fits into a materialist picture of the world. The word materialism to many suggests a hard-headed, tough-minded view: the world is smaller than you thought, less special or less sacred, just atoms bumping into each other. Atoms bumping into each other are indeed quite important, but I do not want to get the story moving with a mood of toughness and restriction in the air. The physical or material world is more than a world of thudding collisions or dry structure. It is a world of energy and fields and hidden influences. We should be ready for ongoing surprises about what it contains.

The approach taken in this book is a biological materialism, but in many ways the heart of my outlook is a broader position, sometimes called monism. Monism is a commitment to an underlying unity in nature, a unity at the most basic levels. Materialism is one kind of monism, as it is committed to the idea that mental phenomena, including subjective experience, are manifestations of more basic activities described in biology, chemistry, and physics. Idealism, the idea that everything is mental, is another kind of monism—it is a different assertion of unity. (An idealist must explain how what seem to be physical objects and goings-on are really manifestations of mind or spirit.) Yet another way of being a monist is to think that both what we call the physical and what we call the mental are manifestations of something else that is basic; this view is called neutral monism. Rather than explaining the mental in physical terms or explaining the physical in mental terms, we explain both the physical and mental in terms of something else. That something else tends to remain rather mysterious. If I was not a materialist I’d be a neutral monist, but that is an outside possibility for me. The way I will proceed is by starting with life—understood in a materialist way—and trying to show how the evolutionary development of living systems can give rise to minds. I want to close—partially, at least—the explanatory gap between mental and physical.

Before we proceed, however, let’s take a closer look at the mental side of the puzzle, and the words we use to describe it. The side of the mind that Nagel tried to point to by saying there is something it’s like… is now often called consciousness. (Nagel himself calls it that.) You are conscious, in this sense, if there is something it feels like to be you. But the term consciousness is often misleading here, as it tends to suggest something quite sophisticated. That phrase something it’s like… is supposed to include the presence of feelings of any kind. There is something it feels like to be you—or a fish, or a moth—if the vaguest, dimmest washes of sensation are part of your life. The fact that the word consciousness suggests more than this tends to cause trouble.

For example, neuroscientists often say that consciousness depends on the cerebral cortex, the folded part at the top of our brains, something found only in mammals and some other vertebrates. Here is a quote from the physician and essayist Oliver Sacks, talking about a patient who had, as a result of a brain infection, lost all ability to hold new events in memory. Sacks asked: What is the relationship of action patterns and procedural memories, which are associated with relatively primitive portions of the nervous system, to consciousness and sensibility, which depend on the cerebral cortex? Sacks is asking a question here, but also stating an assumption: that consciousness and sensibility depend on the cerebral cortex. Does Sacks mean that if someone or something lacks a cerebral cortex, they will lack consciousness in its here-I-am richness, but might still have some feelings? Or does he think that without a cortex the lights are completely off, and any such being would have no experience at all, even if it could manage some behaviors? Most animals, especially most of the animals in this book, do not have a cerebral cortex. Do they have experience of a different kind from us, or no experience at all?

Some people do think that without a cortex there can be no experience at all. Perhaps we will be pushed to a view like this in the end, but I doubt it. We need to continually avoid falling into the habit of thinking that all forms of experience must be human-like in various ways. When the word consciousness is used for the very broad idea of felt experience, it is easy to go astray. But many people do now use the word consciousness, or some modification of it (phenomenal consciousness), in this very broad manner. I am not going to be fussy about the words, and no terminologies are perfect. In some ways, sentience is a good term for the broader concept. We can ask: Which animals are sentient? This is, or might be, different from asking which ones are conscious. But sentience is often used for particular kinds of experience—for pleasure and pain and related experiences that include a valuation, good or bad. Those experiences are certainly important, and it probably makes sense to think that they can exist without sophisticated kinds of consciousness. But these may not be the only kinds of basic or simple experience. In a later chapter, I will look at the possibility that sensory and evaluative sides of experience are somewhat distinct—registering what is going on might be distinct from evaluating whether it is good or bad. Sentience is not usually used for the sensory side of